Klitschko's withdrawal, and an announcement by Poroshenko late on Friday that he would stand in the May 25 election, sets up a battle between the man known as the 'Chocolate King' and Ukraine's fiery former prime minister, Yulia Tymoshenko.

"The only chance of winning is to nominate one candidate from the democratic forces," Klitschko told a meeting of his UDAR (Punch) party, saying he backed Poroshenko.